The present invention relates generally to image recording/playback systems, and, more particularly, to a circuit for detecting recording and playback modes in an image recording/playback system, e.g., in a video tape recorder (VTR).
Presently available VTRs, e.g., 8 mm VTRs, have an automatic track finding (ATF) function. The ATF function is facilitated by recording four pilot signals at four different frequencies f1, f2, f3, and f4, respectively, on one track of the magnetic video tape. The ATF function is implemented by means of a servo control mechanism which detects the pilot signals and determines the precise frequency thereof. Typically, the pilot signals will have frequencies f1, f2, f3, and f4 of 102 kHz, 119 kHz, 165 kHz, and 149 kHz, respectively.
With reference now to FIG. 1, there can be seen a block diagram of a conventional circuit for discriminating recording and playback modes in a VTR, whose operation is as follows. More particularly, a video playback signal read from a video tape being played back is applied to a bandpass filter 10. The video playback signal includes a luminance signal, a chrominance signal, and an audio signal. The bandpass filter 10 is comprised of four sub-bandpass filters (not shown) each having a different passband centered on the frequency of a respective one of the four pilot signals.
During playback, only one of the four pilot signals will pass through the bandpass filter 10, depending on the particular recording mode in which the video playback signal was recorded, and the playback mode in which the video playback signal is being played back. The output of the bandpass filter 10, which can be considered to be the selected pilot signal, is applied to a counter 20, which functions to generate an output signal having a frequency which is proportional to the frequency of the selected pilot signal. Additional circuitry (not shown) is responsive to the output of the counter 20 for generating a mode discrimination signal indicative of the recording mode in which the video playback signal was recorded, and the playback mode in which the video playback signal is being played back, in a manner more fully described below.
More particularly, with reference now to FIGS. 2A-2D, if the video playback signal was recorded in a standard play (SP) mode and is being played back in a long play (LP) mode, the frequency of the mode discrimination signal output by the counter 20 will be 7.5 Hz. Alternatively, if the video playback signal was recorded in an LP mode and is being played back in an SP mode, the frequency of the mode discrimination signal will be 30 Hz. Finally, if the video playback signal is recorded in the same mode (either LP or SP) that it is being played back, the mode discrimination signal will be 15 Hz.
The above-described conventional recording/playback mode discrimination circuit suffers from the following drawbacks and shortcomings. More particularly, due to the close frequency spacing between adjacent pilot signals, it is difficult to precisely filter out the frequency of the selected pilot signal. Further, because a number of counter stages are necessary for evaluating the frequency bandpassed by the bandpass filter 10, the circuit is unduly complex and unnecessarily large in size.
Based on the above, it can be appreciated that there presently exists a need in the art for a recording/playback mode discrimination circuit which eliminates the above-identified drawbacks and shortcomings of the presently available recording/playback mode discrimination circuits. The present invention fulfills this need.